I have the 8320. With no voltage increase, and only multiplier I was able to hit 4.1-4.2ghz stable and that was on a crappier board. Once I go a better board with better VRM's and up'd my cooling, I can hit 4.6 no problem fully stable. I'm sure I could push it further with hours of tweaking and crap to hit .7 or 4.8, but I'm happy where I'm at. It is a great chip and at that price, good to jump on.
Obviously the best overall is the 9000 series since they are faster, but the heat and power they need and since only a few motherboards support them, coupled with the 4x the price of a 8320, they are not worth it.